Multi-piece solid golf ball

ABSTRACT

In a multi-piece solid golf ball having a core, at least one intermediate layer encasing the core and a cover of at least one layer encasing the intermediate layer, each intermediate layer is formed primarily of a resin material and each layer of the cover is formed primarily of an ionomer resin. The cover has a Shore D material hardness of not more than 58, and the material hardness of the cover is higher than the material hardness of the intermediate layer. The intermediate layer and the cover have a combined thickness of not more than 2.1 mm. The cover has a smaller thickness than the intermediate layer, the ratio (a)/(b) of the cover thickness (a) to the intermediate layer thickness (b) being at least 0.5 and less than 1.0.

BACKGROUND OF THE INVENTION

The present invention relates to a multi-piece solid golf ball having a core, an intermediate layer and a cover formed as successive layers. More specifically, the invention relates to a multi-piece solid golf ball having an excellent flight performance which is intended for use by amateur golfers.

Numerous golf balls with three-piece constructions wherein, as described below, an intermediate layer is interposed between a core and a cover and each layer possesses a specific hardness and thickness have hitherto been disclosed as solid golf balls for addressing the needs of amateur golfers having relatively low head speeds of about 35 to 40 m/s when striking the ball with a driver (W#1). “Amateur golfers” refers herein to players who have lower head speeds than professionals and other skilled golfers, for whom the ball tends to rise poorly after being struck, and for whom the use of drivers having a somewhat large loft is generally regarded as preferable.

For example, prior-art disclosures on multiple-piece solid golf balls having a total cover thickness of 3.0 mm or less include the following: U.S. Pat. Nos. 7,086,967, 7,270,611, 7,273,424, 7,201,671, 7,288,031, 7,445,566, 7,563,180, 7,270,614 and 7,377,864.

However, in the foregoing disclosures, there remains room for improvement in the flight performance as golf balls for amateur golfers. There also remains room for improvement in the feel of the ball at impact and in ball controllability in the short game.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a multi-piece solid golf ball which achieves an even better flight performance when used by amateur golfers, and which also has a good feel at impact and a good controllability in the short game.

As a result of intensive investigations, the inventor has discovered that, in order to give the amateur golfer a competitive edge when playing golf, by creating a multi-piece solid golf ball in which the cover and the intermediate layer are formed to relatively small thicknesses, the thickness ratio therebetween is optimized and the cover and the intermediate layer are conferred with suitable material hardnesses, the distance traveled by the ball on shots with a driver (W#1) by an amateur golfer can be increased. In addition, the inventor has found that such a golf ball is capable of having a good feel with a suitably soft touch, and also has a good controllability in the short game.

Accordingly, the invention provides the following multi-piece solid golf balls.

[1] A multi-piece solid golf ball comprising a core, at least one intermediate layer encasing the core and a cover of at least one layer encasing the intermediate layer, wherein each intermediate layer is formed primarily of a resin material; each layer of the cover is formed primarily of an ionomer resin; the cover has a Shore D material hardness of not more than 58; the cover has a higher material hardness than the intermediate layer; the intermediate layer and the cover have a combined thickness of not more than 2.1 mm; and the cover has a smaller thickness than the intermediate layer, the ratio (a)/(b) of the cover thickness (a) to the intermediate layer thickness (b) being at least 0.5 and less than 1.0. [2] The multi-piece solid golf ball of [1], wherein the ratio (a)/(b) of the cover thickness (a) to the intermediate layer thickness (b) is at least 0.7 and not more than 0.9. [3] The multi-piece solid golf ball of [1], wherein the ratio (c)/(b) of the core diameter (c) to the intermediate layer thickness (b) is at least 30 and not more than 50, and the ratio (c)/(a) of the core diameter (c) to the cover thickness (a) is at least 40 and not more than 60. [4] The multi-piece solid golf ball of [1], wherein the thickness of the intermediate layer is from 0.8 to 1.2 mm. [5] The multi-piece solid golf ball of [1], wherein the thickness of the cover is from 0.5 to 1.0 mm. [6] The multi-piece solid golf ball of [1], wherein the resin material forming the intermediate layer has a melt index (MI) of at least 3 g/10 min and the resin material forming the cover has a melt index (MI) of at least 3 g/10 min. [7] The multi-piece solid golf ball of [1], wherein the cover has a Shore D material hardness of not more than 54.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic sectional view showing a multi-piece solid golf ball (three-layer construction) according to the invention.

FIG. 2 is a front view of a golf ball showing the dimple pattern used on the balls in the examples.

DETAILED DESCRIPTION OF THE INVENTION

The invention is described in greater detail below.

The multi-piece solid golf ball of the invention, as shown in FIG. 1, is a golf ball G having a core 1, an intermediate layer 2 encasing the core, and a cover 3 encasing the intermediate layer. A large number of dimples D are formed on the surface of the cover. The core 1, the intermediate layer 2 and the cover 3 are not limited to single layers, and may each be formed of a plurality of two or more layers.

In the invention, the core diameter, while not subject to any particular limitation, is preferably at least 38.5 mm, more preferably at least 38.7 mm, and even more preferably at least 39.0 mm. The core diameter has no particular upper limit, but is preferably not more than 41.4 mm, more preferably not more than 39.7 mm, and even more preferably not more than 39.4 mm. At a core diameter outside of this range, the initial velocity of the ball may decrease or the feel at impact may worsen.

The core has a deflection when a load is applied thereto, i.e., a deflection (mm) when compressed under a final load of 1,275.9 N (130 kgf) from an initial load state of 98.1 N (10 kgf), which, although not particularly limited, is preferably at least 2.5 mm, more preferably at least 3.0 mm, and even more preferably at least 3.2 mm. The core deflection has no particular upper limit, but is preferably not more than 6.0 mm, more preferably not more than 5.0 mm, and even more preferably not more than 4.2 mm. If this value is too small, i.e., if the core is too hard, the spin rate may rise excessively, resulting in a less than satisfactory distance, and the feel on full shots may be too hard. On the other hand, if the above value is too large, i.e., if the core is too soft, the ball rebound may become too small, resulting in a less than satisfactory distance, and the feel on full shots may be too soft. Also, the durability to cracking on repeated impact may worsen.

The core has a surface hardness, expressed as a JIS-C hardness value, which, although not subject to any particular limitation, is preferably at least 70, more preferably at least 75, and even more preferably at least 80. The JIS-C surface hardness has no particular upper limit, but is preferably not more than 93, more preferably not more than 88, and even more preferably not more than 85. If this value is too low, the spin rate may rise excessively or the rebound may decrease, resulting in a less than satisfactory distance. On the other hand, if the above value is too large, the feel at impact may become hard or the durability to cracking on repeated impact may worsen.

The core has a center hardness, expressed as a JIS-C hardness value, which, although not subject to any particular limitation, is preferably at least 55, more preferably at least 57, and even more preferably at least 59. The JIS-C center hardness has no particular upper limit, but is preferably not more than 68, more preferably not more than 65, and even more preferably not more than 64. If this value is too low, the durability to cracking on repeated impact may worsen. On the other hand, if the above value is too large, the spin rate may rise excessively, resulting in a less than satisfactory distance.

The cross-sectional hardness at a position midway between the surface and the center of the core, expressed as a JIS-C hardness value, is preferably at least 62, more preferably at least 65, and even more preferably at least 67. The cross-sectional hardness has no particular upper limit, but is preferably not more than 81, more preferably not more than 77, and even more preferably not more than 72. At a cross-sectional hardness outside of this range, the spin rate may increase, resulting in a poor flight, or the durability to cracking on repeated impact may worsen.

It is preferable for the core hardness to increase gradually from the center to the surface of the core, and for the difference therebetween to be at least 15 JIS-C hardness units. The hardness difference is more preferably at least 16, with the upper limit being preferably not more than 40, and more preferably not more than 35. If the above hardness difference is too small, the spin rate-lowering effect on shots with a W#1 may be inadequate, which may result in a less than satisfactory distance. On the other hand, if the above hardness difference is too large, the initial velocity of the ball on actual shots may become lower, resulting in a less than satisfactory distance, or the durability to cracking on repeated impact may worsen. It is desirable for the above core hardness profile to be one having a linear slope from the center toward the surface.

In addition, in the core hardness profile, the difference between the cross-sectional hardness A at a position midway between the center and the surface of the core and the average B of the hardnesses at the core center and core surface, i.e., the value A-B, is preferably ±5 or less, more preferably ±4 or less, and even more preferably ±3 or less. If this value A-B is too large, the spin rate-lowering effect on W#1 shots, may be inadequate, resulting in a less than satisfactory distance.

The material making up the core having the desired properties mentioned above is not subject to any particular limitation, although the core can be formed using a rubber composition which includes, for example, a co-crosslinking agent, an organic peroxide, an inert filler and an organosulfur compound. Polybutadiene is preferably used as the base rubber of such a rubber composition.

The polybutadiene has a cis-1,4 bond content on the polymer chain of typically at least 60 wt %, preferably at least 80 wt %, more preferably at least 90 wt %, and most preferably at least 95 wt %. Too low a cis-1,4 bond content among the bonds on the molecule may lead to a lower resilience.

The polybutadiene has a 1,2-vinyl bond content on the polymer chain of typically not more than 2%, preferably not more than 1.7%, and more preferably not more than 1.5%, of the bonds on the polymer chain. Too high a 1,2-vinyl bond content may lead to a lower resilience.

To obtain the rubber composition in a molded and vulcanized form having a high resilience that increases the distance traveled by the ball, the polybutadiene used in the invention is preferably one synthesized with a rare-earth catalyst or a Group VIII metal compound catalyst. Polybutadiene synthesized with a rare-earth catalyst is especially preferred.

Such rare-earth catalysts are not subject to any particular limitation. Exemplary rare-earth catalysts include those made up of a combination of a lanthanide series rare-earth compound with an organoaluminum compound, an alumoxane, a halogen-bearing compound and an optional Lewis base.

Examples of suitable lanthanide series rare-earth compounds include halides, carboxylates, alcoholates, thioalcoholates and amides of atomic number 57 to 71 metals.

The use of a neodymium catalyst in which a neodymium compound serves as the lanthanide series rare-earth compound is particularly advantageous because it enables a polybutadiene rubber having a high cis-1,4 bond content and a low 1,2-vinyl bond content to be obtained at an excellent polymerization activity. Suitable examples of such rare-earth catalysts include those mentioned in JP-A 11-35633, JP-A 11-164912 and JP-A 2002-293996.

To enhance the resilience, it is preferable for the polybutadiene synthesized using the lanthanide series rare-earth compound catalyst to account for at least 10 wt %, preferably at least 20 wt %, and more preferably at least 40 wt %, of the rubber components.

Rubber components other than the above-described polybutadiene may be included in the base rubber insofar as the objects of the invention are attainable. Illustrative examples of rubber components other than the above-described polybutadiene include other polybutadienes and other diene rubbers, such as styrene-butadiene rubber, natural rubber, isoprene rubber and ethylene-propylene-diene rubber.

Examples of co-crosslinking agents include unsaturated carboxylic acids and the metal salts of unsaturated carboxylic acids.

Specific examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The metal salts of unsaturated carboxylic acids, while not subject to any particular limitation, are exemplified by the above-mentioned unsaturated carboxylic acids neutralized with a desired metal ion. Specific examples include the zinc and magnesium salts of methacrylic acid and acrylic acid. The use of zinc acrylate is especially preferred.

The unsaturated carboxylic acid and/or metal salt thereof is included in an amount, per 100 parts by weight of the base rubber, of generally at least 10 parts by weight, preferably at least 15 parts by weight, and more preferably at least 18 parts by weight, but generally not more than 60 parts by weight, preferably not more than 50 parts by weight, more preferably not more than 45 parts by weight, and most preferably not more than 40 parts by weight. Too much may make the core too hard, giving the ball an unpleasant feel on impact, whereas too little may lower the rebound.

The organic peroxide may be a commercially available product, suitable examples of which include Percumyl D (produced by NOF Corporation), Perhexa 3M (NOF Corporation) and Luperco 231XL (Atochem Co.). These may be used singly, or two or more may be used together.

The amount of organic peroxide included per 100 parts by weight of the base rubber is generally at least 0.1 part by weight, preferably at least 0.3 part by weight, more preferably at least 0.5 part by weight, and most preferably at least 0.7 part by weight. The upper limit is generally not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Too much or too little organic peroxide may make it impossible to achieve a ball having a good feel, durability and rebound.

Examples of suitable inert fillers include zinc oxide, barium sulfate and calcium carbonate. These may be used singly, or two or more may be used together.

The amount of inert filler included per 100 parts by weight of the base rubber is generally at least 1 part by weight, and preferably at least 5 parts by weight, but generally not more than 100 parts by weight, preferably not more than 80 parts by weight, and more preferably not more than 60 parts by weight. Too much or too little inert filler may make it impossible to achieve a proper weight and a good rebound.

In addition, an antioxidant may be included if necessary. Illustrative examples of suitable commercial antioxidants include Nocrac NS-6 and Nocrac NS-30 (both available from Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (available from Yoshitomi Pharmaceutical Industries, Ltd.). These may be used singly, or two or more may be used together.

The amount of antioxidant included per 100 parts by weight of the base rubber is more than 0, preferably at least 0.05 part by weight, and more preferably at least 0.1 part by weight, but generally not more than 3 parts by weight, preferably not more than 2 parts by weight, more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight. Too much or too little antioxidant may make it impossible to achieve a good rebound and durability.

An organosulfur compound may be included in the core so as to enhance the rebound and increase the initial velocity of the golf ball. It is recommended that thiophenols, thionaphthols, halogenated thiophenols, or metal salts thereof be included here as the organosulfur compound. Illustrative examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, the zinc salt of pentachlorothiophenol, and diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides having 2 to 4 sulfurs. The use of diphenyldisulfide or the zinc salt of pentachlorothiophenol is especially preferred.

The amount of the organosulfur compound included per 100 parts by weight of the base rubber is preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and even more preferably at least 0.2 part by weight. If too little is included, a rebound-improving effect cannot be expected. The upper limit in the amount of organosulfur compound included per 100 parts by weight of the base rubber is preferably not more than 5 parts by weight, more preferably not more than 3 parts by weight, and even more preferably not more than 2.5 parts by weight. If too much is included, an even further rebound-improving effect (particularly on shots with a W#1) cannot be expected, the core may become too soft, and the feel may worsen.

It is desirable to produce the core by using an ordinary mixing apparatus such as a Banbury mixer or a roll mill to mix the core composition containing the above ingredients, compression-molding or injection-molding the mixed composition using a core-forming mold, then suitably heating and curing the molded body at a temperature sufficient for the crosslinking agent and the co-crosslinking agent to act, generally from about 130° C. to about 170° C., and especially 150 to 160° C., for a period of from 10 to 40 minutes, and especially 12 to 20 minutes, so as to achieve the intended hardness profile.

Next, the intermediate layer is described.

The intermediate layer has a material hardness, expressed as a Shore D hardness value (measured with a type D durometer in accordance with ASTM D2240; the same applies below), which, although not subject to any particular limitation, is preferably not more than 55, more preferably not more than 53, and even more preferably not more than 51. The Shore D material hardness of the intermediate layer has a lower limit value of at least 40, and preferably at least 45. If the intermediate layer is too soft, the spin rate on full shots may rise excessively, resulting in a less than satisfactory distance, and the durability to cracking under repeated impact may worsen. On the other hand, if the intermediate layer is too hard, the durability to cracking on repeated impact may worsen, and the spin rate on full shots may rise, resulting in a less than satisfactory distance. Moreover, in such cases, the feel at impact may worsen.

The intermediate layer has a thickness of preferably at least 0.8 mm, more preferably at least 0.9 mm, and even more preferably at least 0.95 mm. The upper limit is preferably not more than 1.2 mm, more preferably not more than 1.1 mm, and even more preferably not more than 1.05 mm. If the intermediate layer is too thin, the durability of the ball to cracking on repeated impact may worsen and the feel at impact may worsen. If the intermediate layer is too thick, the spin rate of the ball when struck with a W#1 may increase, as a result of which the distance may be less than satisfactory.

The intermediate layer material is not particularly limited; various types of thermoplastic resins or thermoplastic elastomers may be used for this purpose. Examples include ionomeric resins, polyester elastomers and urethane resins. In particular, the chief material making up the intermediate layer is preferably one which includes:

(A) a base resin containing

-   -   (a-1) an olefin-unsaturated carboxylic acid random copolymer         and/or a metal ion neutralization product of an         olefin-unsaturated carboxylic acid random copolymer, and     -   (a-2) an olefin-unsaturated carboxylic acid-unsaturated         carboxylic acid ester random terpolymer and/or a metal ion         neutralization product of an olefin-unsaturated carboxylic         acid-unsaturated carboxylic acid ester random terpolymer     -   in proportions of (a-1)/(a-2)=100/0 to 0/100 (weight ratio); and         (B) a non-ionomeric thermoplastic elastomer         in proportions of A/B=100/0 to 50/50 (weight ratio); and more         preferably one which includes:

-   100 parts by weight of a resin component composed of above base     resin (A) and above non-ionomeric thermoplastic elastomer (B) in     proportions of A/B=100/0 to 50/50 (weight ratio);

-   (C) from 5 to 120 parts by weight of an organic fatty acid and/or an     organic fatty acid derivative having a molecular weight of between     280 and 1500; and

-   (D) from 0.1 to 17 parts by weight of a basic inorganic metal     compound capable of neutralizing unneutralized acid groups on the     resin component and component C.

It is preferable to use, as the olefin in above components (a-1) and (a-2), an olefin in which the number of carbons is generally at least 2 but not more than 8, and preferably not more than 6. Specific examples include ethylene, propylene, butene, pentene, hexene, heptene and octene. Ethylene is especially preferred.

Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.

The unsaturated carboxylic acid ester in above component (a-2) is exemplified by lower alkyl esters of the above unsaturated carboxylic acids. Illustrative examples include methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, methyl acrylate, ethyl acrylate, propyl acrylate and butyl acrylate. The use of butyl acrylate (n-butyl acrylate, i-butyl acrylate) is especially preferred.

The olefin-unsaturated carboxylic acid random copolymer of above component (a-1) and the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer of above component (a-2) (these are sometimes referred to collectively below as “random copolymers”) can each be obtained by using a known method to random copolymerize the above-described olefin, unsaturated carboxylic acid and, where necessary, unsaturated carboxylic acid ester.

It is desirable for the above random copolymers to have regulated unsaturated carboxylic acid contents (acid contents). In this case, the content of unsaturated carboxylic acid in component (a-1) is generally at least 4 wt %, preferably at least 6 wt %, more preferably at least 8 wt %, and even more preferably at least 10 wt %, but generally not more than 30 wt %, preferably not more than 20 wt %, more preferably not more than 18 wt %, and most preferably not more than 15 wt %. The content of unsaturated carboxylic acid in component (a-2) is generally at least 4 wt %, preferably at least 6 wt %, and more preferably at least 8 wt %, but generally not more than 15 wt %, preferably not more than 12 wt %, and more preferably not more than 10 wt %.

If the unsaturated carboxylic acid content in above component (a-1) and/or component (a-2) is too low, the ball rebound may decrease, whereas if it is too high, the processability of the resin material may decrease.

The metal ion neutralization product of the olefin-unsaturated carboxylic acid random copolymer of above component (a-1) and the metal ion neutralization product of the olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester random terpolymer of above component (a-2) (these are referred to collectively below as “metal ion neutralization products of the random copolymers”) can be obtained by neutralizing some or all of the acid groups on the respective above random copolymers with metal ions.

Illustrative examples of metal ions for neutralizing acid groups in the above random copolymers include Na⁺, K⁺, Li⁺, Zn⁺⁺, Cu⁺⁺, Mg⁺⁺, Ca⁺⁺, Co⁺⁺, Ni⁺⁺ and Pb⁺⁺. Of these, Na⁺, Li⁺, Zn⁺⁺ and Mg⁺⁺ are preferred. From the standpoint of improving resilience, the use of Na⁺ or Mg⁺⁺ is even more preferred.

The method for obtaining metal ion neutralization products of the above random copolymers using such metal ions may involve neutralization by adding, for example, compounds such as formates, acetates, nitrates, carbonates, bicarbonates, oxides, hydroxides and alkoxides of the above-mentioned metal ions to the above random copolymers having acid groups. In the present invention, no particular limitation is imposed on the degree of neutralization of the acid groups by these metal ions.

Commercially available products may be used as above component (a-1) and above component (a-2). Examples of commercial products that may be used as the random copolymer in above component (a-1) include Nucrel 1560, Nucrel 1214 and Nucrel 1035 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), and Escor 5200, Escor 5100 and Escor 5000 (all products of ExxonMobil Chemical). Examples of commercial products that may be used as the metal ion neutralization products of a random copolymer in above component (a-1) include Himilan 1554, Himilan 1557, Himilan 1601, Himilan 1605, Himilan 1706 and Himilan AM7311 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 7930 (E.I. DuPont de Nemours & Co.), and Iotek 3110 and Iotek 4200 (ExxonMobil Chemical). Examples of commercial products that may be used as the random copolymer in above component (a-2) include Nucrel AN4311 and Nucrel AN4318 (both products of DuPont-Mitsui Polychemicals Co., Ltd.), and Escor ATX325, Escor ATX320 and Escor ATX310 (all products of ExxonMobil Chemical). Examples of commercial products that may be used as the metal ion neutralization product of a random copolymer in above component (a-2) include Himilan 1855, Himilan 1856 and Himilan AM7316 (all products of DuPont-Mitsui Polychemicals Co., Ltd.), Surlyn 6320, Surlyn 8320, Surlyn 9320 and Surlyn 8120 (all products of E.I. DuPont de Nemours & Co.), and Iotek 7510 and Iotek 7520 (both products of ExxonMobil Chemical). These may be used singly, or two or more may be used together.

Examples of sodium-neutralized ionomeric resins, which are preferred as the metal ion neutralization products of the above random copolymers, include Himilan 1605, Himilan 1601 and Surlyn 8120.

The amount of above component (a-2), as a proportion of the combined amount of components (a-1) and (a-2), is generally at least 0 wt %, and preferably at least 50 wt %, with the upper limit being generally 100 wt % or less.

The above-mentioned non-ionomeric thermoplastic elastomer (B) is a component which is preferably included so as to further improve both the feel of the golf ball at impact and the rebound. In the present invention, the base resin (A) and the non-ionomeric thermoplastic elastomer (B) are sometimes referred to collectively as “the resin components.” Such non-ionomeric thermoplastic elastomers (B) are exemplified by olefin-type elastomers, styrene-type elastomers, polyester-type elastomers, urethane-type elastomers and polyamide-type elastomers. From the standpoint of further increasing the rebound, it is preferable to use an olefin-type elastomer or a polyester-type elastomer. A commercially available product may be used as this type of component B. Illustrative examples include the olefin-type elastomer Dynaron (JSR Corporation) and the polyester-type elastomer Hytrel (DuPont-Toray Co., Ltd.). These may be used singly, or two or more may be used together.

The upper limit in the proportion of the above resin components accounted for by component B is generally not more than 50 wt %, and preferably not more than 40 wt %. If component B accounts for more than 50 wt % of the above resin components, the respective components may have a lower compatibility, which may markedly lower the durability of the golf ball.

Component C in the invention is an organic fatty acid and/or derivative thereof having a molecular weight of at least 280 but not more than 1500. Component C has a much smaller molecular weight than the above resin components and is preferably included because it is a component that suitably adjusts the melt viscosity of the mixture and, in particular, helps to enhance the flow properties.

The organic fatty acid serving as above component C has a molecular weight of generally at least 280, preferably at least 300, more preferably at least 330, and even more preferably at least 360, but generally not more than 1500, preferably not more than 1000, more preferably not more than 600, and even more preferably not more than 500. If the molecular weight is too small, the heat resistance may decrease. On the other hand, if the molecular weight is too large, it may not be possible to improve the flow properties.

It is preferable to use as the organic fatty acid of component C an unsaturated organic fatty acid containing a double bond or triple bond on the alkyl moiety, or a saturated organic fatty acid in which the bonds on the alkyl moiety are all single bonds. The number of carbons in one molecule of the organic fatty acid is generally at least 18, preferably at least 20, more preferably at least 22, and even more preferably at least 24, but generally not more than 80, preferably not more than 60, more preferably not more than 40, and even more preferably not more than 30. Too few carbons, in addition to possibly resulting in a poor heat resistance, may also, by making the acid group content relatively high, lead to excessive interactions with acid groups present in the resin component, thereby diminishing the flow-improving effect. On the other hand, too many carbons increases the molecular weight, as a result of which a distinct flow-improving effect may not be achieved.

Illustrative examples of the organic fatty acid of component C in the present invention include stearic acid, 12-hydroxystearic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, arachidic acid and lignoceric acid. Of these, stearic acid, arachidic acid, behenic acid and lignoceric acid are preferred. Behenic acid is especially preferred.

The organic fatty acid derivative of component C is exemplified by metallic soaps in which the proton on the acid group of the organic fatty acid has been replaced with a metal ion. Examples of the metal ion include Na⁺, Li⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Mn⁺⁺, Al⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺, Cu⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. Of these, Ca⁺⁺, Mg⁺⁺ and Zn⁺⁺ are especially preferred.

Specific examples of the organic fatty acid derivative of component C include magnesium stearate, calcium stearate, zinc stearate, magnesium 12-hydroxystearate, calcium 12-hydroxystearate, zinc 12-hydroxystearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate. Of these, magnesium stearate, calcium stearate, zinc stearate, magnesium arachidate, calcium arachidate, zinc arachidate, magnesium behenate, calcium behenate, zinc behenate, magnesium lignocerate, calcium lignocerate and zinc lignocerate are preferred. These may be used singly, or two or more may be used together.

The amount of component C included per 100 parts by weight of the above resin components (components A and B) is generally at least 5 parts by weight, preferably at least 10 parts by weight, more preferably at least 15 parts by weight, and even more preferably at least 18 parts by weight, but generally not more than 120 parts by weight, preferably not more than 80 parts by weight, more preferably not more than 60 parts by weight, and even more preferably not more than 50 parts by weight. If the amount of component C included is too small, the melt viscosity may become excessively low, reducing the processability. On the other hand, if the amount of component C is too high, the durability may decrease. It should be noted that the cover material has a component C content which, unlike that mentioned above, is from 0.1 to 10 parts by weight per 100 parts by weight of the resin components. This is described in detail later in the specification.

In the present invention, use may also be made of, as a mixture of the above-described base resin (A) and the above-described component C, a known metallic soap-modified ionomer (see, for example, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Disclosure WO 98/46671).

Component D in the present invention is a basic inorganic metal compound capable of neutralizing unneutralized acid groups in the resin components and component C. If component D is not included, such as in cases where a metallic soap-modified ionomeric resin is used alone, during mixture under applied heat, the metallic soap and unneutralized acid groups present in the ionomeric resin will undergo exchange reactions, generating a large amount of fatty acid that vaporizes, potentially giving rise to problems such as molding defects, reduced paint film adhesion, and a decrease in the resilience of the resulting molded material. In the present invention, component D is preferably included so as to resolve such problems.

It is preferable for component D to be a compound having a high reactivity with the resin components and containing no organic acids in the reaction by-products. Illustrative examples of the metal ion in component D include Li⁺, Na₊, K⁺, Ca⁺⁺, Mg⁺⁺, Zn⁺⁺, Al⁺⁺⁺, Ni⁺⁺, Fe⁺⁺, Fe⁺⁺⁺, Cu⁺⁺, Mn⁺⁺, Sn⁺⁺, Pb⁺⁺ and Co⁺⁺. These may be used singly, or two or more may be used together. Known basic inorganic fillers containing these metal ions may be used as component D. Illustrative examples include magnesium oxide, magnesium hydroxide, magnesium carbonate, zinc oxide, sodium hydroxide, sodium carbonate, calcium oxide, calcium hydroxide, lithium hydroxide and lithium carbonate. In particular, a hydroxide or a monoxide is recommended. Calcium hydroxide and magnesium oxide, which have high reactivities with the base resin, are preferred.

The amount of component D included per 100 parts by weight of the resin components is generally at least 0.1 part by weight, preferably at least 0.5 part by weight, more preferably at least 1 part by weight, and even more preferably at least 2 parts by weight, but generally not more than 17 parts by weight, preferably not more than 15 parts by weight, more preferably not more than 13 parts by weight, and even more preferably not more than 10 parts by weight. If the amount of component D included is too small, improvements in the thermal stability and resilience may not be observed. On the other hand, if it is too large, the presence of excess basic inorganic metal compound may have the opposite effect of lowering the heat resistance of the golf ball material. It should be noted that the cover material has a component D content which, unlike that mentioned above, is from 0.1 to 5 parts by weight per 100 parts by weight of the resin components. This is described in detail later in the specification.

The mixture obtained by mixing together above components A to D has a degree of neutralization, based on the total amount of acid groups in the mixture, of generally at least 50 mol %, preferably at least 60 mol %, more preferably at least 70 mol %, and even more preferably at least 80 mol %. With such a high degree of neutralization, even in cases where, for example, a metallic soap-modified ionomeric resin is used, exchange reactions between the metallic soap and unneutralized acid groups present in the ionomeric resin are less likely to arise during mixture under heating, thereby reducing the likelihood of declines in thermal stability, moldability and resilience.

In addition to above components A to D, various additives such as pigments, dispersants, antioxidants, ultraviolet absorbers and light stabilizers may also be included within the intermediate layer material. These additives are used in an amount per 100 parts by weight of the resin components made up of component A and component B which, although not subject to any particular limitation, is generally at least 0.1 part by weight, preferably at least 0.5 part by weight, and more preferably at least 1 part by weight, but generally not more than 10 parts by weight, preferably not more than 6 parts by weight, and more preferably not more than 4 parts by weight.

The resin-based mixed material for the intermediate layer may be obtained by mixing the respective above components A to D under applied heat. For example, they may be obtained by mixture, using an internal mixer such as a kneading-type twin-screw extruder, a Banbury mixer or a kneader, at a heating temperature of from 150 to 250° C. Alternatively, direct use may be made of a commercial product, illustrative examples of which include those available under the trade names HPF 1000, HPF 2000 and HPF AD1027, as well as the experimental material HPF SEP1264-3, all produced by E.I. DuPont de Nemours & Co.

From the standpoint of, for example, ensuring flow properties that are particularly suitable for injection molding and thus improving the moldability, it is preferable to regulate the melt index of the above intermediate layer material. In this case, the melt index (MI), as measured in accordance to ASTM D1238 at a test temperature of 190° C. and a test load of 21.2 N (2.16 kgf), is preferably at least 1 g/10 min, more preferably at least 2 g/10 min, and even more preferably at least 3 g/10 min. It is recommended that the upper limit be set to preferably not more than 20 g/10 min, more preferably not more than 15 g/10 min, and even more preferably not more than 10 g/10 min. If the melt index is too low, molding may be difficult to carry out or the sphericity of the intermediate layer-covered sphere may decrease, giving rise to variability in the cover thickness, which may result in greater variability in the flight of the ball. On the other hand, if the melt index is too high, the durability to cracking on repeated impact may worsen.

Next, the Shore D material hardness of the cover used in the invention must be not more than 58, and is preferably not more than 56, and more preferably not more than 54. The lower limit is preferably at least 45, and more preferably at least 50. If the cover is softer than the above range, the ball may be too receptive to spin or the rebound may be inadequate, resulting in a shorter distance, or the scuff resistance may worsen. On the other hand, if the cover is too hard, the ball may have a poor durability to cracking on repeated impact, or may have a poor feel at impact in the short game or when struck with a putter.

The cover has a thickness of preferably at least 0.5 mm, more preferably at least 0.6 mm, and even more preferably at least 0.7 mm, but preferably not more than 1.0 mm, more preferably not more than 0.9 mm, and even more preferably not more than 0.8 mm. At a cover thickness smaller than the above range, the ball may have a poor durability to cracking on repeated impact. On the other hand, if the cover is thicker than the above range, the spin rate of the ball may increase excessively on shots with a W#1 and the rebound may decrease, possibly resulting in a less than satisfactory distance.

The cover material is formed primarily of an ionomer resin. Use can be made of, specifically, an ionomer such as Surlyn (available from E.I. DuPont de Nemours & Co.), and Himilan and AM7331 (both from DuPont-Mitsui Polychemicals Co., Ltd.). A non-ionomeric resin material, such as those available under the trade name “Nucrel” from DuPont-Mitsui Polychemicals Co., Ltd., may be included in the cover material in an amount, based on the total amount of the cover material, of not more than 50 wt %, preferably not more than 30 wt %, and more preferably not more than 20 wt %. If the amount of non-ionomeric resin material included is greater than this range, the durability to cracking on repeated impact may worsen, the ability to bond with paint may worsen, and the cover may damage more easily when struck with an iron.

From the standpoint of, for example, ensuring flow properties that are particularly suitable for injection molding, and thus a good moldability, it is preferable to regulate the melt index of the above cover material. In this case, the melt index (MI), as measured in accordance to ASTM D1238 at a test temperature of 190° C. and a test load of 21.2 N (2.16 kgf), is typically at least 3 g/10 min, preferably at least 3.2 g/10 min, and more preferably at least 3.4 g/10 min. It is recommended that the upper limit be set to preferably not more than 10 g/10 min, and more preferably not more than 5 g/10 min. If the melt index is too low, molding may be difficult to carry out or the sphericity of the ball may decrease, which may result in greater variability in the flight of the ball. On the other hand, if the melt index is too high, the durability to cracking on repeated impact may worsen.

In addition to the above resin components, various additives may be optionally included in the above-described resin materials for the intermediate layer and the cover. Examples of such additives include pigments, dispersants, antioxidants, ultraviolet absorbers, ultraviolet stabilizers, mold release agents, plasticizers, and inorganic fillers (e.g., zinc oxide, barium sulfate, titanium dioxide).

Combined Thickness of Intermediate Layer and Cover

In this invention, the combined thickness of the intermediate layer and the cover must be not more than 2.1 mm, and is preferably not more than 2.0 mm, and more preferably not more than 1.85 mm. The lower limit in the combined thickness is preferably at least 1.3 mm, more preferably at least 1.5 mm, and even more preferably at least 1.65 mm. If the combined thickness is too large, the spin rate on full shots will increase, resulting in a less than satisfactory distance. On the other hand, if the combined thickness is too small, the spin rate on full shots may increase, resulting in a less than satisfactory distance and the durability to cracking on repeated impact may worsen.

Ratio of Cover Thickness to Intermediate Layer Thickness

In this invention, it is critical for the ratio of the cover thickness to the intermediate layer thickness to be suitably set within a given range. Specifically, the ratio (a)/(b) of the cover thickness (a) to the intermediate layer thickness (b) must be at least 0.5, and is preferably at least 0.7, and more preferably at least 0.75, with the upper limit being not more than 1.0, preferably not more than 0.95, and even more preferably not more than 0.90. At a value outside the above range, a suitable spin rate is not obtained, resulting in a less than satisfactory distance.

Ratio of Core Diameter to Cover Thickness

In this invention, although not subject to any particular limitation, it is preferable for the ratio of the core diameter to the cover thickness to be suitably set within a given range. Specifically, the ratio (c)/(a) of the core diameter (c) to the cover thickness (a) is preferably at least 40, more preferably at least 42, and even more preferably at least 44, with the upper limit being preferably not more than 60, more preferably not more than 58, and even more preferably not more than 56. At a value outside the above range, a suitable spin rate may not be obtained, which may result in a less than satisfactory distance.

Ratio of Core Diameter to Intermediate Layer Thickness

In this invention, although not subject to any particular limitation, it is preferable for the ratio of the core diameter to the intermediate layer thickness to be suitably set within a given range. Specifically, the ratio (c)/(b) of the core diameter (c) to the intermediate layer thickness (b) is preferably at least 30, more preferably at least 33, and even more preferably at least 35, with the upper limit being preferably not more than 50, more preferably not more than 47, and even more preferably not more than 45. At a value outside the above range, a suitable spin rate may not be obtained, which may result in a less than satisfactory distance.

Relationship Between Intermediate Layer Material Hardness and Cover Material Hardness

In the present invention, it is critical for the following relationship between the intermediate layer material hardness and the cover material hardness to be satisfied:

cover material hardness>intermediate layer material hardness.

By designing the golf ball in such a way that the material hardness of the cover is higher than the material hardness of the intermediate layer, the flight performance can be further enhanced, enabling a crisp feel at impact to be obtained.

In the practice of the invention, numerous dimples may be formed on the surface of the cover. The dimples arranged on the cover surface, while not subject to any particular limitation, number preferably at least 280, more preferably at least 300, and even more preferably at least 320, but preferably not more than 360, more preferably not more than 350, and even more preferably not more than 320. If the number of dimples is higher than the above range, the ball will tend to have a low trajectory, which may shorten the distance of travel. On the other hand, if the number of dimples is smaller than the above range, the ball will tend to have a high trajectory, as a result of which an increased distance may not be achieved.

The geometric arrangement of the dimples on the ball may be, for example, octahedral or icosahedral. In addition, the dimple shapes may be of one, two or more types suitably selected from among not only circular shapes, but also various polygonal shapes, such as square, hexagonal, pentagonal and triangular shapes, as well as dewdrop shapes and oval shapes. The diameter (in polygonal shapes, the lengths of the diagonals), although not subject to any particular limitation, is preferably set to from 2.5 to 6.5 mm. In addition, the depth, although not subject to any particular limitation, is preferably set to from 0.08 to 0.30 mm.

The value V₀, defined as the spatial volume of a dimple below the flat plane circumscribed by the dimple edge, divided by the volume of the cylinder whose base is the flat plane and whose height is the maximum depth of the dimple from the base, although not subject to any particular limitation, may be set to from 0.35 to 0.80 in this invention.

From the standpoint of reducing aerodynamic resistance, the ratio SR of the sum of individual dimple surface areas, each defined by the flat plane circumscribed by the edge of a dimple, with respect to the surface area of a hypothetical sphere were the ball surface to have no dimples thereon, although not subject to any particular limitation, is preferably set to from 60 to 90%.

The ratio VR of the sum of the spatial volumes of individual dimples, each formed below the flat plane circumscribed by the edge of a dimple, with respect to the volume of a hypothetical sphere were the ball surface to have no dimples thereon, although not subject to any particular limitation, may be set to from 0.6 to 1% in this invention.

In this invention, by setting the above V₀, SR and VR values in the foregoing ranges, the aerodynamic resistance is reduced, in addition to which a trajectory which provides a good distance readily arises, enabling the flight performance to be enhanced.

The golf ball of the invention, which can be manufactured so as to conform with the Rules of Golf for competitive play, is preferably produced to a ball diameter which is of a size that will not pass through a ring having an inside diameter of 42.672 mm, but is not more than 42.80 mm, and to a weight of generally from 45.0 to 45.93 g.

In the invention, the surface of the golf ball cover may be subjected to various types of treatment, such as surface preparation, stamping and painting, in order to enhance the design and durability of the ball.

As was explained above, the multi-piece solid golf ball of the invention further increases the flight performance, enabling amateur golfers to play golf very competitively. Moreover, the multi-piece solid golf ball of the invention enables a good, solid feel to be obtained at impact and also has a good controllability in the short game.

EXAMPLES

Working Examples of the invention and Comparative Examples are given below by way of illustration, and not by way of limitation.

Examples 1 to 3, Comparative Examples 1 to 5

Solid cores were produced by preparing rubber compositions according to the formulations shown in Table 1 below, then molding and vulcanizing the compositions at 155° C. for 15 minutes.

TABLE 1 Core formulation (parts by Example Comparative Example weight) 1 2 3 1 2 3 4 5 Polybutadiene 100 100 100 100 100 100 100 100 Zinc acrylate 29.9 27.5 25.1 25.1 29.9 29.9 28.5 29.9 Zinc salt of 0.5 0.5 0.5 0.5 0.5 0.5 0 0.5 pentachloro- thiophenol Peroxide 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 Zinc oxide 21.9 22.8 24.0 31.9 21.9 21.9 22.2 18.2

Details on the above core materials are given below. The numbers in the table represent parts by weight.

-   Polybutadiene: Available under the trade name “BR 730” from JSR     Corporation. -   Peroxide: A mixture of 1,1-di(t-butylperoxy)cyclohexane and silica,     available under the trade name “Perhexa C-40” from NOF Corporation. -   Antioxidant: 2,2-Methylenebis(4-methyl-6-butylphenol), available     under the trade name “Nocrac NS-6” from Ouchi Shinko Chemical     Industry Co., Ltd.

Formation of Intermediate Layer and Cover

Next, intermediate layers were formed by injection-molding resin materials of the compositions shown in Table 2 over the respective solid cores fabricated as described above, thereby giving in each Example a sphere composed of a solid core encased by an intermediate layer (an intermediate layer-covered sphere). Next, a cover was formed by injection-molding over this sphere a resin material of the composition shown in Table 2, thereby giving a multi-piece solid golf ball having a three-layer construction composed of a solid core encased by an intermediate layer and a cover. Dimples in the arrangement shown in FIG. 2 were formed at this time on the surface of the ball cover in each of the Examples of the invention and the Comparative Examples. Details on the dimples are given in Table 3.

TABLE 2 No. 1 No. 2 No. 3 No. 4 No. 5 Surlyn 7930 47.0 Surlyn 6320 38.5 60 Surlyn 9945 50 Surlyn 8940 50 Surlyn 8120 100 AN4318 14.5 N035C 40 Hytrel 4001 15 Magnesium stearate 69 Magnesium oxide 0.8 T-8295 75 T-8290 25 Polyethylene wax 1.5 Isocyanate compound 9 Titanium oxide 2.1 2.1 5 3.5 Numbers in the table indicate parts by weight.

The above trade names are explained below.

The trade names of the chief materials mentioned in the table are as follows.

-   Surlyn: Ionomers available from E.I. DuPont de Nemours and Co. -   AN4318 and N035C: Available under the trade name “Nucrel” from     DuPont-Mitsui Polychemicals Co., Ltd. -   Hytrel: Polyester elastomers available from DuPont-Toray Co., Ltd. -   Magnesium oxide: Available as “Kyowamag MF150” from Kyowa Chemical     Industry Co., Ltd. -   T-8290 and T-8295: MDI-PTMG type thermoplastic polyurethanes     available from DIC Bayer Polymer under the trade name “Pandex” -   Polyethylene wax: Available under the trade name “Sanwax 161P” from     Sanyo Chemical Industries, Ltd. -   Isocyanate compound: 4,4′-Diphenylmethane diisocyanate

TABLE 3 Number of Diameter Depth No. dimples (mm) (mm) V₀ SR VR 1 12 4.6 0.15 0.47 81 0.783 2 234 4.4 0.15 0.47 3 60 3.8 0.14 0.47 4 6 3.5 0.13 0.46 5 6 3.4 0.13 0.46 6 12 2.6 0.10 0.46 Total 330

Dimple Definitions

-   Diameter: Diameter of flat plane circumscribed by edge of dimple. -   Depth: Maximum depth of dimple from flat plane circumscribed by edge     of dimple. -   V₀: Spatial volume of dimple below flat plane circumscribed by     dimple edge, divided by volume of cylinder whose base is the flat     plane and whose height is the maximum depth of dimple from the base. -   SR: Sum of individual dimple surface areas, each defined by the flat     plane circumscribed by the edge of a dimple, as a percentage of the     surface area of a hypothetical sphere were the ball to have no     dimples on the surface thereof (units: %). -   VR: Sum of spatial volumes of individual dimples formed below flat     plane circumscribed by the edge of a dimple, as a percentage of the     volume of a hypothetical sphere were the ball to have no dimples on     the surface thereof (units: %).

For each of the golf balls obtained, physical properties such as the thicknesses and hardnesses of the respective layers, and also the flight performance and feel at impact of the balls, were evaluated by the methods described below. The results are presented in Tables 4 and 5. All of the measurements were carried out in a 23° C. atmosphere.

(1) Core Deflection (mm)

The core or an intermediate layer-covered sphere was compressed in a 23.9±1° C. atmosphere at a rate of 10 mm/s, and the deflection (mm) upon applying a final load of 1,275.9 N (130 kgf) from an initial load state of 98.1 N (10 kgf) was measured. In each case, the average of measurements taken on ten balls (N=10) was determined.

(2) Center Hardness, Surface Hardness and Cross-Sectional Hardness of Core

The surface of the core being spherical, the durometer indenter was set substantially perpendicular to this spherical surface, and the Shore D hardness was measured with a type D durometer in accordance with ASTM-2240. The JIS-C hardness was measured in accordance with JIS K6301-1975.

The center hardness and the cross-sectional hardness midway between the core surface and the core center were determined by cutting the core into two with a fine cutter, and measuring the hardnesses at specific positions in accordance with the respective hardness standards.

(3) Material Hardnesses of Intermediate Layer and Cover (Hardnesses of Sheet-Type Molded Materials)

The material that forms each layer was molded into a sheet having a thickness of 2 mm and held for 2 weeks at 23±2° C., following which the Shore D hardness was measured with a type D durometer in accordance with ASTM-2240.

(4) Melt Index (MI) Values of Intermediate Layer Material and Cover Material

The melt index (MI) values of the materials used to form the respective layers were measured in accordance with ASTM D1238 (test temperature, 190° C.; test load, 21.2 N (2.16 kgf)).

(5) Flight Performance on Shots with a Driver The distance was measured by mounting a driver (W#1) manufactured by Bridgestone Sports Co., Ltd. (TourStage ViQ, 2012 model; loft, 11.5°) on a golf swing robot and striking the ball at a head speed (HS) of 38 m/s. The flight performance was rated according to the criteria indicated below. In addition, the spin rate of the ball immediately after being struck in the same way was measured with an apparatus for measuring initial conditions.

-   -   Good: Total distance was 178 m or more     -   NG: Total distance was less than 178 m

(6) Feel

The feel of the ball when hit with a driver (W#1) by ten amateur golfers having head speeds (HS) of 35 to 40 m/s was sensory evaluated under the following criteria.

-   -   Good: At least seven out of the ten golfers rated the ball as         having a good feel     -   NG: Three or fewer of the ten golfers rated the ball as having a         good feel         (A “good feel” refers to a feel having a suitably soft touch; a         feel which is too soft or too hard is a bad feel.)

In addition, the feel on shots with a putter was sensory evaluated under the following criteria.

-   -   Good: At least seven out of the ten golfers rated the ball as         having a good feel     -   NG: Three or fewer of the ten golfers rated the ball as having a         good feel         (7) Approach Shots with a Sand Wedge

A sand wedge (SW) was mounted on a golf swing robot, and the spin rate (rpm) when the ball was struck at a head speed (HS) of 20 m/s was measured. The club used was a TourStage X-WEDGE manufactured by Bridgestone Sports Co., Ltd. (2011 model; loft, 56′). The flight performance was rated according to the following criteria.

-   -   Good: The spin rate on approach shots was 5,700 rpm or more     -   NG: The spin rate on approach shots was less than 5,500 rpm

(8) Durability to Cracking on Repeated Impact

The ball was repeatedly hit at a head speed of 40 m/s with a W#1 club mounted on a golf swing robot. The balls in the respective Examples were rated as shown below relative to an arbitrary durability index of 100 for the number of shots taken with the ball in Example 3 before the initial velocity fell to or below 97% of the average initial velocity for the first ten shots. The average value for N=3 balls was used as the basis for evaluation in each Example.

-   -   Good: Durability index was 95 or more     -   NG: Durability index was less than 95

TABLE 4 Example Comparative Example 1 2 3 1 2 3 4 5 Core Diameter (mm) 39.1 39.1 39.1 36.5 39.1 39.1 39.1 39.1 Weight (g) 36.6 36.6 36.6 31.1 36.6 36.6 36.6 35.9 Deflection under 10-130 kgf 3.4 3.7 4.0 4.0 3.4 3.4 3.4 3.4 loading (mm) Surface hardness JIS-C 83 81 80 79 83 83 83 83 Shore D 55 54 52 52 55 55 55 55 Hardness 19.55 mm JIS-C 71 69 67 — 71 71 71 71 from center: A Shore D 46 44 43 — 46 46 46 46 (between core surface and center) Hardness 18.25 mm JIS-C — — — 67 — — — — from center: A Shore D — — — 43 — — — — (between core surface and center) Center hardness JIS-C 63 61 60 60 63 63 63 63 Shore D 40 39 38 38 40 40 40 40 Surface − Center JIS-C 20 20 19 19 20 20 20 20 Shore D 15 15 15 14 15 15 15 15 (Surface + Center)/2: B (JIS-C) 73 71 70 70 73 73 73 73 B − A (JIS-C) 2 2 3 3 2 2 2 2 Intermediate Material (type) No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 No. 2 layer Specific gravity 0.95 0.95 0.95 0.95 0.95 0.95 0.95 0.95 Material hardness (Shore D) 50 50 50 50 50 50 50 50 Melt index (at 190° C.) 3.1 3.1 3.1 3.1 3.1 3.1 3.1 3.1 Thickness (mm) 1.0 1.0 1.0 1.7 1.0 0.7 1.0 1.0 Intermediate Diameter (mm) 41.1 41.1 41.1 39.9 41.1 40.5 41.1 41.1 layer-covered Weight (g) 41.4 41.4 41.4 38.5 41.4 39.9 41.4 40.7 sphere Cover Material (type) No. 1 No. 1 No. 1 No. 1 No. 3 No. 1 No. 4 No. 5 Specific gravity 0.96 0.96 0.96 0.96 0.96 0.96 0.98 1.14 Material hardness (Shore D) 53 53 53 53 45 53 63 53 Melt index (at 190° C.) 3.4 3.4 3.4 3.4 1.0 3.4 3.2 — Thickness (mm) 0.8 0.8 0.8 1.4 0.8 1.1 0.8 0.8 Ball Diameter (mm) 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Weight (g) 45.5 45.5 45.5 45.5 45.5 45.5 45.5 45.5 Cover thickness/Intermediate layer thickness 0.8 0.8 0.8 0.82 0.8 1.57 0.8 0.8 Core diameter/Cover thickness 48.9 48.9 48.9 26.1 48.9 35.5 48.9 48.9 Core diameter/Intermediate layer thickness 39.1 39.1 39.1 21.5 39.1 55.9 39.1 39.1

TABLE 5 Example Comparative Example 1 2 3 1 2 3 4 5 Flight W#1 Total distance 178.2 179.6 179.4 175.6 174.5 176.9 179.5 178.0 (HS = 38 m/s) (m) Spin rate (rpm) 3,100 3,018 2,936 3,236 3,188 3,182 3,015 3,153 Distance (m) good good good NG NG NG good good Approach shots with sand good good good good good good NG good wedge Feel on shots with W#1 good good good good good good good good Feel on shots with putter good good good good good good NG good Durability to cracking good good good good good good NG NG on repeated impact

As is apparent from Table 5, the respective Comparative Examples were inferior to the Working Examples of the invention in the following ways.

In Comparative Example 1, the combined thickness of the cover and the intermediate layer was large and the spin rate on shots with a driver (W#1) was high, as a result of which a good distance was not achieved.

In Comparative Example 2, the cover was softer than the intermediate layer and the rebound on shots with a W#1 was low, as a result of which a good distance was not achieved.

In Comparative Example 3, the cover was thicker than the intermediate layer, the spin rate on shots with a W#1 was high, and the rebound was low, as a result of which a good distance was not achieved.

In Comparative Example 4, the cover was hard and the spin rate in the short game was insufficient, lowering the controllability. In addition, the ball had a hard feel on shots with a putter and the durability to cracking on repeated impact was poor.

In Comparative Example 5, the cover material was composed primarily of urethane, adhesion between the cover and the intermediate layer was poor, and the durability to cracking on repeated impact was poor. 

1. A multi-piece solid golf ball comprising a core, at least one intermediate layer encasing the core and a cover of at least one layer encasing the intermediate layer, wherein each intermediate layer is formed primarily of a resin material; each layer of the cover is formed primarily of an ionomer resin; the cover has a Shore D material hardness of not more than 58; the cover has a higher material hardness than the intermediate layer; the intermediate layer and the cover have a combined thickness of not more than 2.1 mm; and the cover has a smaller thickness than the intermediate layer, the ratio (a)/(b) of the cover thickness (a) to the intermediate layer thickness (b) being at least 0.5 and less than 1.0.
 2. The multi-piece solid golf ball of claim 1, wherein the ratio (a)/(b) of the cover thickness (a) to the intermediate layer thickness (b) is at least 0.7 and not more than 0.9.
 3. The multi-piece solid golf ball of claim 1, wherein the ratio (c)/(b) of the core diameter (c) to the intermediate layer thickness (b) is at least 30 and not more than 50, and the ratio (c)/(a) of the core diameter (c) to the cover thickness (a) is at least 40 and not more than
 60. 4. The multi-piece solid golf ball of claim 1, wherein the thickness of the intermediate layer is from 0.8 to 1.2 mm.
 5. The multi-piece solid golf ball of claim 1, wherein the thickness of the cover is from 0.5 to 1.0 mm.
 6. The multi-piece solid golf ball of claim 1, wherein the resin material forming the intermediate layer has a melt index (MI) of at least 3 g/10 min and the resin material forming the cover has a melt index (MI) of at least 3 g/10 min.
 7. The multi-piece solid golf ball of claim 1, wherein the cover has a Shore D material hardness of not more than
 54. 